Synthesis of a polyester, such as polyethylene terephthalate, is usually achieved by an esterification reaction followed by a polycondensation reaction, a transesterification reaction followed by a polycondensation reaction, and a reaction of alkylene oxide with a dicarboxylic acid followed by a prepolymerization reaction and then a polycondensation reaction. For example, terephthalic acid (TPA) is reacted with ethylene oxide to form bis(2-hydroxyethyl) terephthalate (BHET), which is subjected to a prepolymerization reaction at a temperature ranging from 245° C. to 270° C. to form a prepolymer, which is then subjected to a polycondensation reaction at a temperature ranging from 270° C. to 290° C. to obtain polyethylene terephthalate.
U.S. Pat. No. 6,350,851 B1 discloses a method of polymerizing deionized bis-β-hydroxyethyl terephthalate, which includes (1) providing polyester production raw materials including bis-β-hydroxyethyl terephthalate containing cations and anions in a total weight of not more than 50 ppm, ethylene glycol and at least one polymerization catalyst, (2) heating the polyester production raw materials without substantially distilling off ethylene glycol, and (3) polycondensing the raw materials by heating under reduced pressure while distilling off ethylene glycol. In the illustrated examples of U.S. Pat. No. 6,350,851 B1, in step (2), purified bis-β-hydroxyethyl terephthalate is heated at 245° C. to obtain an oligomer, which is polycondensed at 280° C. to obtain polyethylene terephthalate.